Atmospheric pollution is a societal problem which is receiving much attention. The major source of such pollution is the extensive use of fossil fuels, although industrial and chemical processes, such as the manufacture of nitric acid, also contribute. The principal pollutants are nitrogen oxides, carbon monoxide, and perhaps to a lesser extent hydrocarbons, sulfur oxides and other objectionable gases and vapors.
Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, viz., nitric oxide (NO) and nitrogen dioxide (NO2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further here since they are well recognized and have led various government authorities to restrict industrial emissions in an attempt to limit the level of the nitrogen oxides in the atmosphere. Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation EQU 2NO+O2=&gt;2NO2.
For purposes of the present invention NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide, and mixtures thereof.
Formation of man-made nitrogen oxides from the elements occurs in the high temperature zones of combustion processes. The internal combustion engine, and coal-, oil-, and gas-fired furnaces, boilers and incinerators, all contribute to NO.sub.x emissions. In general, fuel-rich combustion mixtures produce exhaust gases with lower contents of NO.sub.x than do lean mixtures. Although the concentrations of NO.sub.x in the exhaust gases produced by combustion usually are low, the aggregate amounts discharged in industrial and/or highly populated areas is adequate to cause problems.
The so-called "stable" nitrogen oxides have in common the somewhat peculiar property that although they are thermodynamically very unstable with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method has been described for inducing this decomposition. A variety of catalysts are known which reduce NO.sub.x to N.sub.2, using carbon monoxide, hydrogen or hydrocarbons in a net reducing environment. Since all three of these reductants are present in normal automobile emissions, this would appear to be a simple matter. Unfortunately, oxygen is also present in such emissions and most catalysts which reduce NO.sub.x will not operate effectively in an oxidizing atmosphere. Instead of reducing NO.sub.x the reductants reduce oxygen. One class of materials, copper-exchanged zeolites, have been used to overcome this problem, and have been shown to be suitable catalysts for reduction of NO.sub.x in automobile engine exhaust containing hydrocarbons which act as reductants. For example, U.S. Pat. No. 4,297,328 discloses concurrent catalytic reduction of oxides of nitrogen and the oxidation of carbon monoxide and hydrocarbons in a gas stream containing a stoichiometric excess of oxidant, over a copper-containing ZSM-5 zeolite. U.S. Pat. No. 5,041,270 discloses NO.sub.x reduction in the presence of hydrocarbons acting as reductant in an oxidizing atmosphere, over a catalyst containing copper loaded on a support. U.S. Pat. No. 5,041,272 discloses NO.sub.x reduction in the presence of excess oxygen, in the presence of organic reductant, e.g., using hydrocarbons over hydrogen form zeolites such as ZSM-5 which are impregnated with a metal such as copper.
Despite the initial effectiveness of such NO.sub.x reducing copper-containing catalysts employed in lean burn exhaust operations, their ultimate service life is severely limited during operations under hydrothermal conditions. Hydrothermal stability of NO.sub.x reduction catalysts is considered in U.S. Pat. No. 4,157,375. This reference discloses the preparation of a zeolite prepared from a calcined honeycomb preform such as kaolin with an aqueous solution of base (e.g., water, NaOH and tetrapropylammonium) to form a monolith containing ZSM-5. However, the resulting catalyst is used for reduction of nitrogen oxides in exhaust gases in the presence of "suitable reducing gas, ammonia, carbon monoxide, hydrogen or the like . . . added in an amount such that the added gas together with any reducing agent present (e.g., carbon monoxide) will be about equal to the stoichiometric amount required for a desired reduction of NO.sub.x " (column 8, lines 14 to 21). The resulting catalyst is suited to operation at temperatures up to 800.degree. C. in the absence of water and up to 700.degree. C. in the presence of substantial amounts of water due to the sensitivity of zeolites to elevated temperature in the presence of steam. No mention is made of utilizing such a catalyst in NO.sub.x reduction employing a hydrocarbon reductant.
All of the above patents are incorporated herein by reference in their entirety.